1. Field of the Invention
This invention relates to a light quantity control device adapted for use in an optical apparatus such as a camera, an observation apparatus, a projection-type television set, or the like.
2. Description of the Related Art
The conventional diaphragm device mounted on a video camera or the like has, as shown by way of example in FIGS. 3 and 4, diaphragm blades 37 and 38 arranged as light quantity control members and a motor 1 for bringing a quantity of light to a desired quantity by driving and controlling the diaphragm blades 37 and 38. The motor 1 is arranged to be controlled by a motor control voltage Vc outputted from a comparison computing circuit 6 arranged to compare a light quantity control signal Vs with a speed control signal. The motor 1 is provided with a rotor magnet 2, a driving coil 3 for rotating the rotor magnet 2, a damping coil 4 for detecting the rotational speed of the rotor magnet 2 and a Hall element 5 for detecting the position of rotation of the rotor magnet 2. The device is arranged as follows: When the rotor magnet 2 rotates, a speed detection signal which is generated by the damping coil 4 is negatively fed back to the comparison computing circuit 6 via an amplification circuit 9. The motor control voltage Vc outputted from the comparison computing circuit 6 is applied to one terminal of the driving coil 3, while a ground level constant voltage is applied to the other terminal of the driving coil 3. The output of the Hall element 5 which detects the rotation position of the rotor magnet 2 by detecting changes in density of the magnetic flux of the rotor magnet 2 is inputted as an aperture value signal Vo to an exposure control circuit (not shown) through a differential amplifier 8.
FIG. 4 shows the mechanical arrangement of the conventional diaphragm device described above. Referring to FIG. 4, a printed circuit board 31 is provided for the motor 1. A motor cap 32 is arranged to serve as a bearing plate for supporting one of shafts of the rotor magnet 2. A yoke 33 which is made of a ferromagnetic material is arranged to form a magnetic circuit in conjunction with the rotor magnet 2. A diaphragm base plate 34 is provided with an aperture part 34a and arranged to have the motor 1 secured thereto. A spring 35 is arranged to drive the diaphragm blades 37 and 38 in one direction through a driving arm 36. The driving arm 36 transmits the driving force of the rotor magnet 2 and that of the spring 35 to the diaphragm blades 37 and 38. A cover 39 is provided with an aperture part 39a. The diaphragm blades 37 and 38 have diaphragm aperture parts 37a and 38a formed respectively therein.
A quantity of light passing through the aperture parts 34a and 39a is controlled by driving the diaphragm blades 37 and 38 with the driving force of the rotor magnet 2 and that of the spring 35 through the driving arm 36 to vary the area of an aperture formed jointly by the diaphragm aperture parts 37a and 38a of the two diaphragm blades 37 and 38.
When the motor control voltage Vc is applied to the driving coil 3, the rotor magnet 2 rotates in the direction of opening the diaphragm aperture (increasing the area of the aperture) to drive the diaphragm blades 37 and 38 to move in the aperture opening direction through the driving arm 36. In closing the diaphragm aperture, the diaphragm blades 37 and 38a are driven by the spring force of the spring 35.
The conventional diaphragm device, however, always necessitates a sufficient torque for overcoming the spring force of the spring 35 in causing the diaphragm blades 37 and 38 to move in the direction of opening the diaphragm aperture. Hence, the device has presented the following problems:
(i) The necessity of a sufficient torque for overcoming the spring force of the spring 35 inevitably causes an increase in size of the motor 1 and, hence, an increase in power consumption, which are contrary to the object of designing. PA1 (ii) In order to always obtain the torque counteracting the spring force of the spring 35, it is necessary to apply a large current to the driving coil 3. This causes a magnetic flux produced from the driving coil 3 to come into the Hall element 5 to lower the accuracy of detection of the position of the rotor 2. Further, the Hall element 5 is affected by an excitation noise which results from the mutual induction of the driving coil 3 and the damping coil 4, degrading the S/N ratio of the aperture value signal Vo, and, as a result, the diaphragm device or the camera tends to be caused to malfunction.